Sunday 15 August 2010 05.35 EDT
First published on Sunday 15 August 2010 05.35 EDT

The UN secretary general, Ban Ki-moon, has urged the world to speed up aid efforts to Pakistan as the country braces itself for further flooding with the waters of the swollen Indus river reaching critical levels.

"This has been a heart-wrenching day for me," Ban said after flying over some of the worst-hit areas. "I will never forget the destruction and suffering I have witnessed today. In the past I have witnessed many natural disasters around the world, but nothing like this."

The UN has appealed for an initial £295m to provide relief, but only 20% of that has so far been given.

"Waves of flood must be met with waves of support from the world," said Ban. "I'm here to urge the world to step up assistance," he said.

Ban has met the prime minister, Yousuf Raza Gilani, and the president, Asif Ali Zardari, who was fiercely condemned for failing to cut short a European visit as the crisis deepened.

In his first comments to the media since returning, Zardari defended the government's response. "The government has responded very responsibly," he said, saying the army, the police and officials were all working to relieve the suffering. "I would appeal to the press to understand the magnitude of the disaster."

With more than 1,600 people confirmed dead and as many as 20 million made homeless, the country is reeling from the scale of the catastrophe brought by torrential monsoon rains.

Gilani said Pakistan now faced challenges similar to those during the 1947 partition of the subcontinent, when as many as 500,000 people were killed. His warning came amid growing fears of social unrest or even a military takeover after the government's shambolic response. "The nation faced the situation successfully at that time of the partition and, God willing, we will emerge successful in this test," he said.

In the southern province of Sindh, 1.6 million people are stranded and half a million have been ordered to abandon their homes. The first case of cholera since the disaster began has been recorded in the northern Swat valley, while survivors across the eastern state of Punjab are sleeping in the open without shelter.

The situation in some areas is threatening to spill over into violence, according to witnesses. An Associated Press correspondent in Sindh reported seeing survivors fighting over food, ripping at each other's clothes and causing such chaos that the distribution had to be abandoned. "The impatience of the people has deprived us of the little food that had come," Shaukat Ali, a flood victim waiting for aid, said.

Although water levels are continuing to rise, offers of international aid remain small compared with recent disasters elsewhere, to the dismay of Pakistan's leaders and aid agencies. Only £96m has been pledged – with £31m of that from the British government – in response to the UN appeal.

International donations in the wake of the Haiti earthquake earlier in the year totalled £1.5bn. David Cameron's criticism of Pakistan as an exporter of terrorism has been blamed in part for the lack of public sympathy, as has Zardari's failure to cancel his visit to Britain.

India yesterday pledged £3.2m, the same amount it gave to Haiti, but not before its apparent hesitancy prompted claims that a recent political spat was overriding its humanitarian imperative. Critics said Pakistan did not hesitate to come to India's aid when the Gujarat earthquake killed 25,000 people in 2001. They said that India's offer was a tiny fraction of its £500m aid budget for this year.

China, which initially offered just £960,000, has now increased its contribution to £4.6m. Britain's Disasters Emergency Committee appeal has raised £12m from public donations.

Zamir Akram, Pakistan's ambassador to the UN, criticised the international response and claimed that the scale of the disaster was only just beginning to become apparent.

Late yesterday, there were no indications that the worst was over. Pakistan's flood control department warned that water in the upper reaches of the Indus was at "very high levels".

At some points along its course, the river was reported to be 15 miles wide, more than 25 times its normal size. Further flooding is now expected in Sindh province, which includes Karachi, the country's largest city.

The discovery of the first cholera case in Mingora, in the Swat valley, confirmed the fears of aid agencies who had been warning of the danger of disease. Cholera can lead to severe dehydration and death without prompt treatment, and containing cholera outbreaks is a priority following floods.

The UN said it feared the case was not isolated, adding that it was now treating 36,000 people as if they were suffering from cholera. Aid agencies warned that 6 million children were at risk of life-threatening diarrhoeal diseases, malnutrition and pneumonia.

A 28-year-old Save the Children worker, unnamed for security reasons, undertook a 30-mile trek to assist aid distribution programmes in the town of Kalam, in the Swat valley, after roads in the area were destroyed.

On his blog, he wrote: "We reached the first town, Adyan, after crossing two hills. The entire shape of the city had changed ‑ the floods created a river that went straight through the middle of town, completely destroying the main market.

"Mud and dust was everywhere, as were huge boulders that the flood had carried right into town. We finally reached a city, Bahrain, which used to be a big tourist destination with lots of hotels, restaurants, and beautiful riverside cafes. I was there five years ago on holiday with my family.

"The city is now unrecognisable. It is like something has taken a huge pile of rocks and mud and thrown it all over the city. The main bazaar is completely destroyed. Three-storey hotels have tumbled down and the main road through the town was covered in five feet of mud.

"In Kalam, 90% of the main market, which provides a livelihood for so many people, was completely destroyed. It looked like it might have 100 years ago: no cars ‑ they had all washed away ‑ no clean water supply, which was previously run by an electric pumping station, now destroyed."